The interaction between myosin and actin, which is directly responsible for the generation of contractile force, has been studied in a wide variety of different muscles, including smooth muscles of the viscera and systemic vasculature, but no studies on the pulmonary vasculature have been reported. Because this interaction and its regulation by Ca 2 ion varies among different types of muscles, it is important to study this interaction with myosin and actin from the pulmonary vessels in order to understand contraction in the pulmonary circulatory system. Our objectives are to isolate myosin from the pulmonary artery and characterize enzymatic activity and subunit composition, to identify proteins which promote or are necessary for the interaction of myosin and actin and those necessary for regulation of this interaction by Ca 2 ion, and to obtain further information about the mechanism by which these proteins influence the actin-myosin interaction. Myosin and actin will be purified from bovine pulmonary artery and myosin will be characterized by its enzymatic activity, its interaction with actin, and its electrophoretic properties. We will then isolate protein components of pulmonary muscle which promote the actin-myosin interaction and its regulation using the actin-activated ATPase of myosin and SDS gel electrophoresis to monitor activity and purity, respectively. We will study the effects of these proteins and Ca 2 ion on the ATPase activity of actomyosin and on the rotational motions of crossbridges of myosin filaments and those of actin filaments by saturation transfer EPR spectroscopy to obtain further insight into the mechanism of regulation of contraction in pulmonary muscles.